Not sure what the problem is asking.
Would you multiply 1080 x 720 to get the total pixels on the
screen, then divide that product by the area of the screen?
The above would give an answer in pixels per square inch,
which I don't think that's PPI means. It should be PPSI. ???
We could calculate the length of a side to obtain the screen area
IF we assume a square screen, but I wouldn't go out on that limb.
We could even calculate the area of the screen if we assume it's in
the Golden Ratio, but again, I don't want to make that assumption.